a(n) = n!*(2*n-5)/2.

Terms

a(0) =3a(1) =36a(2) =300a(3) =2520a(4) =22680a(5) =221760a(6) =2358720a(7) =27216000a(8) =339292800a(9) =4550515200a(10) =65383718400